Recently Issued Accounting Standards - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues	$ 33,681	$ 28,100	$ 64,881	$ 56,400
NACoal
Disaggregation of Revenue [Line Items]
Revenues	33,681	28,100	64,881	56,400
NACoal | Operating segments
Disaggregation of Revenue [Line Items]
Revenues			64,881	56,400
Consolidated operations - long-term contracts | NACoal
Disaggregation of Revenue [Line Items]
Revenues	29,960	24,878	57,983	50,079
Royalty | NACoal
Disaggregation of Revenue [Line Items]
Revenues	3,721	3,222	6,898	6,321
Goods transferred at a point in time | NACoal
Disaggregation of Revenue [Line Items]
Revenues	20,174	17,492	37,195	34,407
Services transferred over time | NACoal
Disaggregation of Revenue [Line Items]
Revenues	$ 13,507	$ 10,608	$ 27,686	$ 21,993